Childhood Dermatomyositis and Polymyositis

Abstract

Childhood dermatomyositis has been recognized as a clinicopathologic entity for approximately two decades. Polymyositis in the pediatric age group, as in adults, is not a singly defined entity, as it may occur with or without evidence of one of the collagen vascular diseases. Because of overlap of some features of both disorders the terms "polymyositis" and "polymyositis—dermatomyositis Complex" have sometimes been used to include both of these disorders. INCIDENCE Polymyositis and dermatomyositis are uncommon disorders; even in a referral center, there may be only two or three cases seen per year. The limited number of epidemiologic studies have included both adults and children and have combined several types of inflammatory myopathy. By this imperfect method, the annual incidence of new cases has varied from 1 to 7.7 per million persons. There is an increased incidence in the second and seventh decades of life, with a lower incidence in the third decade. Most reports of dermatomyositis have shown a preponderance of girls. Onset most commonly occurs at school age, but children in their second year of life have been affected. PATHOGENESIS Studies of muscle biopsies from patients with juvenile dermatomyositis have demonstrated that the basic lesion involves the endothelial cells of intramuscular capillaries, arterioles, and veins.